Deposition of aluminum, magnesium or aluminum/magnesium alloys on materials consisting of base metals is a convenient way of protecting such materials from corrosion. At the same time, they are provided with a decorative coating. To this end, the protective metal layer is predominantly deposited on the material by means of electroplating. Advantageously, the aluminum, magnesium or aluminum/magnesium layer is coated on the material with no coating of metallic intermediate layers between said metal layer and said material. If intermediate layers have been coated between the material and the surface layer of aluminum, magnesium or aluminum/magnesium alloy, there is a risk of contact corrosion due to the coated intermediate layer. In addition, thermal problems may arise due to the different expansion coefficients of the surface layer and intermediate layer.
Electrolytes found useful in the prior art include fused-salt electrolytes, such as electrolytes containing aluminum halides or aluminum alkyl complexes. A common feature in all of these electrolytic systems is that the material has to be cleaned prior to coating the surface thereof. Thus, particularly with materials consisting of base metals forming an oxide layer, there is a problem in that such oxide layers must be completely removed prior to coating. If the surface of such materials has not been completely cleaned, impurities or residues of the oxide layer of the metal constituting the material, which adhere to the surface, result in impaired adhesion of a metal layer subsequently coated by electrolysis. Furthermore, it is possible that no metal layer at all is coated on those spots where impurities are present on the surface, because said impurities normally are electrically non-conductive, thereby preventing electrolytic deposition at that spot. This inevitably gives rise to corrosion problems of the finished coated material at those spots where coating of the metal layer is incomplete.
DE-C3-22 60 191 describes a method of preparing materials made of electroconductive materials. In this method, the last process step, which serves to shape the materials, and in which a new bare surface is formed on the material, is performed in a suitable inert gas or inert fluid medium, with exclusion of atmospheric oxygen and moisture. This method turned out to be disadvantageous in that, particularly when using an inert fluid medium that covers the surface of the material and therefore might enter the coating electrolyte, the electrolyte is subsequently contaminated or hydrolyzed by same. When using inert gas media, one problem arising in large-scale industrial applications is that an inert gas atmosphere absolutely free of oxygen cannot be accomplished in practice. Traces of oxygen present in the inert gas atmosphere immediately oxidize the bare metal surface of the material, thus giving rise to the above-described loss in quality of the metal layer subsequently coated by means of electroplating. If, as described in DE-C3-22 60 191, the bare surface is achieved by means of a mechanical procedure, such as milling, cutting, sawing or drilling, or by means of massive deformation of the material using e.g. rolling or wire drawing, extrusion or other procedures, such procedures will give rise to an increased production tolerance of the finished material. As a result, materials produced according to the above method are not suitable for applications where highly constant quality and manufacturing are required.
DE-AS-12 12 213 describes the pretreatment of a material in a protective gas atmosphere. Alternatively, the oxide layer on the surface of the material can be removed by connecting the material as anode prior to deposition of the aluminum layer in the electrolyte which is produced from sodium fluoride and triethylaluminum. Thereafter, the current is reversed, and aluminum is deposited on the material. Disadvantageously, it has been found that the electrolyte can only be used in the deposition of aluminum on materials. Deposition of magnesium or aluminum/magnesium layers is not possible because the presence of halide ions in the electrolyte would result in immediate formation of insoluble magnesium halide compounds during anodic polarity, preventing deposition of magnesium or aluminum/magnesium on the material. The magnesium halides being formed would immediately stop the current in the electrolyte by blocking the electrodes.
DE-AS-21 22 610 describes a method for the anodic pretreatment of light metals for the electrodeposition of aluminum. Cleaning of the components is effected by treating the light metal materials in a fused electrolyte, thereby subjecting the materials to anodic load. The light metal materials cleaned in this way, being wetted with electrolyte, i.e. still loaded with fused electrolyte, are immersed in an aluminizing cell. During this operation, the possibility of oxygen still reaching the pretreated material, re-oxidizing it on the surface thereof, cannot be excluded. Furthermore, the aluminizing electrolyte is contaminated by the surface treatment electrolyte, which is a fused electrolyte. Only in those cases where the material consists of beryllium or aluminum, the fused electrolyte used in surface treatment by anodic oxidation of the material can also be used in the electrodeposition of aluminum on the beryllium or aluminum material. The fused electrolyte described in DE-AS-21 22 610 is only suitable for pretreatment of beryllium or aluminum materials in order to effect subsequent coating thereof with aluminum in the same fused electrolyte. The fused electrolyte is not suitable for electrodeposition of aluminum, magnesium or aluminum/magnesium layers on other materials.
DE-A1-198 55 666 describes an electrolyte suitable for the deposition of aluminum/magnesium alloy layers. The organoaluminum electrolyte disclosed therein contains K[AlEt4] or Na[Et3Al—H—AlEt3], Na[AlEt4], as well as trialkylaluminum. The electrolyte can be present in the form of a toluene solution. Electrolytic deposition of aluminum/magnesium alloy layers from the electrolyte described therein is effected using a soluble aluminum anode and a likewise soluble magnesium anode, or using an anode made of aluminum/magnesium alloy. In the described method, the electrolyte composition is adjusted by pre-electrolysis in such a way that the deposited layer has the desired aluminum/magnesium ratio. Alternatively, Mg[AlEt4]2 can also be added to the electrolyte. Thus, the teaching of DE-A1-198 55 666 is that the ratio of aluminum and magnesium in the deposited aluminum/magnesium layer strongly depends on the concentration ratio of magnesium and aluminum in the electrolyte. As in all prior art methods, great care must be taken in the pretreatment of the materials to be coated, because impurities in the surface of the materials caused by oxidation or other influences result in reduced quality of the metal layer deposited by electroplating.